Moving pictures are stored as video. The moving pictures represent a sequence of still images, that when displayed give the appearance of motion. The images may be stored as “frames” or “fields”. A frame represents a single still image. By contrast, a field typically represents every second horizontal line (or row) in a still image. More generally, a frame can be thought of as the combination of two fields that form an image.
Fields are typically referred to as odd and even, with odd field containing odd lines of a frame, and even fields containing even lines of a frame.
Analog television in North America is broadcast in accordance with the NTSC standard, and uses interlaced fields. The NTSC standard calls for about sixty (exactly 59.94) fields to be presented each second. Sequential fields are received and presented. The human eye perceives two sequential fields as a single frame.
Most cinema films are filmed at a rate of twenty-four images per second. Accordingly, cinema films have historically been converted for analog NTSC television using a process known as 3:2 pull-down. This conversion is more particularly illustrated in FIG. 1. As illustrated a sequence of film images, represented as frames ABCDE . . . is divided into even and odd fields, and one field of every second frame is repeated. The resulting field sequence is AOAEAOBEBOCECOCEDODEEOEEEO. . . Thus, fields from every second frame are presented in three fields, and fields formed from every second other frame are presented for only two fields. Twenty four frames are thus converted into sixty interlaced fields.
In Europe, analog television is transmit in accordance with the PAL or SECAM standards. These standards call for 50 interlaced fields per second. Cinema films are converted for analog PAL/SECAM television using a process known as 2:2 pull-down. This conversion is also illustrated in FIG. 1. As illustrated a sequence of film frames ABCDE . . . is used to form corresponding even and odd fields. Each even and odd field is shown once every second frame. The resulting field sequence is AOAEBOBECOCEDODEEOEE . . . . Twenty four frames are thus converted into about fifty (i.e. forty eight) interlaced fields.
Newer television, computer and similar displays, however no longer display interlaced video. Instead such displays display the video progressively, one line after the next. Accordingly, newer video output devices, such as for example digital versatile disk (DVD) players, computer games and the like, output video progressively, line by line.
Often, video to be presented by such progressive scan devices comes from video provided as fields. For example, many DVDs still store video as fields of MPEG (or MPEG2) data, that must be de-interlaced. Moreover, many such sources store the video as fields in film mode (e.g. 3:2 pull-down or 2:2 pull-down). Progressive scan devices must be able to accurately assemble progressive scan frames from the interlaced data. They must therefore be able to detect the field sequence (often referred to as cadence) to correctly combine the fields to form frames. To this end, many sources (such as DVDs) are coded with flags that are intended to indicate whether stored video is stored in film mode or not. Unfortunately, video is often poorly edited: video is cut apart and reassembled in a way that destroys or alters the sequence. Similarly, video is often transferred or broadcast without these flags. In short, the flags cannot be relied upon.
As a result, other field sequence detectors (or cadence detectors) are known. Some detectors compare the difference between pixels in adjacent fields. If the source video is stored in film mode, the differences will follow a predictable pattern. Many of these detection circuits, however, cannot robustly distinguish between noise, slow movements, content that overlays interlaced and film mode material, high frequencies, and conventional interlaced content.
Other techniques include the analysis of motion vector data from field to field. Such techniques, however, are quite complex.
Accordingly, there remains a need for a field sequence detector and method that can easily and quickly detect the presence of film mode video.